1. Field of the Invention
In general, the present invention relates to an apparatus for optically recording and playing back information into and from a disk. In particular, the present invention relates to signal processing by an optical disk apparatus.
2. Description of the Related Art
In the conventional optical disk apparatus, during a playback operation, a light beam having a fixed intensity is radiated from a light source. The light is reflected by a disk with a reflection intensity that varies due to an optical interference effect between light spots formed on the disk by the light beam, and marks on the disk that represent information stored on the disk. The reflected light is received and converted into an electrical signal to be detected and output later.
In this system, the playback electrical signal includes disk noise, laser noise, amplifier noise, and other kinds of noise which inevitably affect the accuracy of the detection of the electrical signal. The effects of the shot noise and the amplifier noise can be reduced effectively by increasing the power of the radiated light. By merely increasing the power of the radiated light, however, the temperature of an area on the disk hit by the radiating light beam increases, inevitably damaging the mark representing information stored on the disk.
In addition, a disk has been proposed which, when manufactured, includes marks that introduce timing information for recording purposes. The timing information is then used for recording information marks on the surface of the disk at predetermined locations.
In a playback operation, pulses radiated by using the timing information as a base are received. In this system, however, marks must be created on the disk in advance at locations at predetermined intervals, making it necessary to divide the region for recording information by the marks provided thereon. As a result, the disk is incompatible with disks manufactured by the conventional technique whereby data are recorded in a contiguous area. Further, the size of the recording area decreases because of the created marks.
The present invention provides a signal processing apparatus that retains compatibility with the conventional continuous servo recording technique, and reduces the amount of noise included in a signal played back from the optical disk, which noise has been a difficult problem for the conventional playback method, without the need to decrease the size of the area for recording data, resulting in a drastically increased signal-to-noise ratio.
The optical disk signal processing apparatus of the present invention employs a light source, an optical system for leading a beam generated by the light source, an optical device for converging the beam output by the rotating system and applying the converged beam to the rotating disk, a signal detecting optical system for detecting a signal reflected by the rotating disk through the optical device, a light detecting instrument for converting light produced by the signal detecting optical system into an electrical signal, and a data detecting circuit. The intensity of the beam radiated by the light source is modulated in synchronism with the timing of data detection so that a playback signal having an improved signal-to-noise ratio can be obtained by integrating the product of a response waveform signal generated by a light beam reflected from an information mark and a light-intensity modulating signal over a fixed period of time.
By providing a correlation between the information mark and the intensity modulation, the intensity of the signal coming from the information mark can be increased and, at the same time, noise not correlated with the light-intensity modulating signal can be suppressed. As a result, the signal-to-noise ratio can be increased effectively.